Crystallinity and rheological behavior of polyetheretherketone during nonisothermal conditions

Solidification models are key during simulation of several industrial processes involving thermoplastics. For simplicity, crystallinity is often not considered within these models, despite it being responsible for the phase transition. Several advanced methods, which consider crystallinity as the onset of solidification have been proposed in literature however, these have primarily been applied to classical homopolymers such as polypropylene (PP). The focus of this study is to develop a model that can capture the rheological response observed during the transition from liquid-like to solid-like behavior in injection molding grade polyetheretherketone (PEEK) due to crystallinity. Isothermal rheological experiments are performed alongside dynamic scanning calorimetry (DSC) characterization to correlate relative crystallinity to the apparent increase in viscosity. The model is extended to nonisothermal processes through the incorporation of the Nakamura model. Nonisothermal crystallization rheology experiments are performed and compared with a simulation of the oscillating rheometer process for validation. The modeled viscosity response and crystallization half-time reproduced the experimental data with sufficient accuracy at low cooling rates, with an error of less than 5% up to cooling rates of 20°C min⁻¹. This shows that the method is an accurate means of obtaining the rheological response during crystallization in a numerical simulation.